1. Field of the Invention
The present invention relates to a robot capable of overcoming obstacles, and more particularly, to an apparatus for moving the center of gravity in a robot to supplement the support provided by the wheels of the robot and directional changes, and a system and method using the apparatus. When wheel-type mobile robots with casters used in home or office environments face obstacles such as doorsills and power cords while moving, they are immobilized when their casters (that are not driven) become hitched on an obstacle. In this case, the present invention uses the center of gravity of the robot to overcome the obstacle.
2. Description of the Related Art
Conventional mobile robots with drive systems employing differentials generally have two driven wheels and one or two non-driven casters, where components are arranged within the robot so that the center of gravity lies between the axes of the drive wheels and the casters, ensuring traction and stability. Since all objects that move on the ground using wheels have their centers of gravity above the contact patches of the wheels, when decelerating, their inertial force causes the contact patches of the front wheels to be the rotational center and the moment to be in the direction of the rear wheels lifting. When the distance between the front wheels and the trailing casters is short compared to the height of unit, as shown in FIG. 1, the moment of inertia is more liable to upset the balance of the unit. Accordingly, in the configuration shown in FIG. 1, the center of gravity is distributed to be biased above the caster by means of an added weight. However, this not only increases the overall weight of the unit, but also concentrates weight over the caster.
Generally, when a mobile robot must pass over an obstacle, the drive wheels are able to roll over the obstacle; but there are many instances in which the casters (that are not driven) are unable to surmount the obstacle and become stuck. This is explained with reference to FIGS. 2a, 2b and 2c. 
FIG. 2a illustrates the force equilibrium at the moment when a drive wheel with a radius r surmounts an obstacle, FIG. 2b illustrates the force equilibrium at the moment when a caster with a radius r surmounts an obstacle, and FIG. 2c illustrates the force equilibrium at the moment when a caster with a radius r2 surmounts an obstacle. A description of the propulsion needed for drive wheels and a caster to overcome an obstacle will now be given.
Referring to FIG. 2a, when the moment M is applied to the axis of the drive wheels, a frictional force F proportional to a reaction force N1 at the contact patches of the drive wheels is generated. At the moment that the drive wheels begin mounting the obstacle, the reaction force is negated, and forces (as shown in the force vector diagram to the right) remain. Here, the frictional force F is as follows.F=W*sin(t)=W*sin(a cos((r−h)/r))
If a greater frictional force is applied against the frictional force F, then acceleration in the direction over the obstacle will be generated to overcome the obstacle. Since the frictional force is always less than a normal force, the angle (t) in the force vector diagram lies within a range between 0° and 45°. Thus: 0°<t<45°.
After the drive wheels satisfy the above conditions and roll over the obstacle, the caster must roll over the obstacle. Assuming that the caster that receives a thrust T1 has a radius r equal to that of the drive wheels, the force vector diagram in FIG. 2b applies.T1=W*tan(t)=W*tan(a cos((r−h)/r))
However, because the caster must rotate in a predetermined direction, increasing the size of the caster necessitates reducing the interior space of the robot. Thus, the actual radius r2 of the caster is always smaller than the radius r of the drive wheels. FIG. 2c shows the radius value r2 of the caster being smaller than the radius r of the drive wheels.T2=W*tan(t2)=W*tan(a cos((r2−h)/r2)
In the range 0°<t<45°, the tan value is always greater than the sin value, and the tan value increases in accordance with an increase in angle. Therefore, relations between the sizes of F, T1, and T2 are as follows.F<T1<T2 
Accordingly, a force greater than the rotating force of the drive wheels is needed for the caster to overcome an obstacle.
When a moment of the wheels is applied with a force in the direction against the caster, the equilibrium of the moment presses the caster against the obstacle, such that a weight W applied on the caster increases from a non-moving state and a thrust T2 increases as well, so that it becomes difficult for the caster to surmount the obstacle. In this case, even with a relatively low obstacle that the robot appears to be able to overcome, due to the caster, the robot is unable to pass over the obstacle.
On the other hand, when the robot applies a moment of the drive wheels in a direction of the caster, the weight applied on the caster is alleviated. However, because the caster is made with a small radius for the sake of miniaturizing the robot's main body, nonetheless, it is difficult for the caster to overcome the obstacle without being lifted.
As a method of overcoming an obstacle by passing over it is disclosed in Japanese Patent Publication No. 2004-321705, entitled “TRANSFER APPARATUS, AND TRAVEL APPARATUS”, which is hereby incorporated by reference, and is thus omitted herefrom. In this disclosure, a linked lift system is used to raise each wheel a certain distance for an elevating effect, which allows the unit to scale stairs, etc. However, this unit cannot easily surmount obstacle on a flat floor surface.
Another apparatus is disclosed in Japanese Patent Publication No. 2005-0053467, entitled “MOBILE ROBOT”, which is hereby incorporated by reference, and is thus omitted herefrom. Here, each wheel has a means for changing its elevation, so that should there be a discrepancy in the height of a floor surface, the wheels may be respectively elevated to maintain level contact with the floor surface. However, this method requires installing complex mechanisms on each wheel, which is unsuitable for indoor robot applications.